Just a note to the group. I lost an omnifet last week. Driving along and then injector #2 was full on! This is on a board I built with 2 - 2 channel fet drivers and 4 omnifets. The other 3 omnifets worked just fine and continue to . There was no electrical issue that I am aware of (no shorts, I was not 'playing with anything nor have I touched it in months). These should be protected against the release spike when the injector closes. Dunno what happened, but it was a bit scary.

This is my first component failure on a MS device since my initial install when I shorted the injector driver (single) fet and filled the engine with gas. THis was figuring how to mate the V2.2 board with my Saab's wiring harness.

I found mine when I probed the gate. The fet driver, IXDN404, was not able to drive the gate any more. My schematic has a pull down resistor on the gate and a resistor inline to the gate from the fet driver. When it failed, I was in 3rd gear going about 15 mph. I pressed on the accelerator and then it was running on 3 cylinders. So, the PW was low then went high when they failed. Mine failed and never came back. These are Bosch -431 HiZ injectors.

I just scoped mine to look at the active clamp (internal to the device) to have a look. It clamps right at 39 volts.If the internal clamp failed, then the device could/would get killed by the fly-back pulse from the injector coil. A secondary clamp in addition would provide protection if the active clamp died. Mine isn't running on the car yet but I imagine that a stuck ON injector isn't a good thing. How far can one go to protect against this? I was considering designing a watchdog to monitor injector pulses assuming that the pulse width never hits 100% ON, this could easily be done.Do other ECUs do anything to protect against a dead driver?Attached is a pic of the working vnd5n07 if anyone is interested.

You can put a diode on a relay to help clamp the flyback, could you do the same at the injectors? Might not be the desired long term solution, but seems like it'd get rid of that 39v spike. Think a 1n4001 would do the trick, or would it need to be stronger/faster than that?

UnaClocker wrote:You can put a diode on a relay to help clamp the flyback, could you do the same at the injectors? Might not be the desired long term solution, but seems like it'd get rid of that 39v spike. Think a 1n4001 would do the trick, or would it need to be stronger/faster than that?

The VND5N07 was purposefully designed to allow the flyback voltage to reach 60V. The higher voltage allows the injectors to close more quickly than if they were clamped to 12V with a 1N400x.

Could injectors be driven with IGBT's? Those things seem a lot more robust than MOSFETs, they take some pretty wicked flyback from the coils from time to time. (Sorry if I'm going off on a tangent here, always trying to expand my knowledge)

Ute_Man wrote:Could injectors be driven with IGBT's? Those things seem a lot more robust than MOSFETs, they take some pretty wicked flyback from the coils from time to time. (Sorry if I'm going off on a tangent here, always trying to expand my knowledge)

Many different devices are suitable for driving injectors. Structurally, IGBT's are similar to a MOSFET driving a bipolar transistor. That alone doesn't make them anymore robust than a MOSFET, though. The IGBT's you're thinking of probably have built-in circuitry to protect against high voltages. The VND5N07 also has built-in circuitry for the same purpose, yet sometimes they fail.

No one is sure that the flyback is what's damaging these VND5N07's anyway.

EFIDART512 wrote:I just scoped mine to look at the active clamp (internal to the device) to have a look. It clamps right at 39 volts.If the internal clamp failed, then the device could/would get killed by the fly-back pulse from the injector coil. A secondary clamp in addition would provide protection if the active clamp died. Mine isn't running on the car yet but I imagine that a stuck ON injector isn't a good thing.

[snip]

Attached is a pic of the working vnd5n07 if anyone is interested.

Has anyone looked at it with a high resolution scope (like an Agilent 3000 or 7000)? There could be a short spike there that goes above 60V and over time wears out the protection circuit (or the FET itself). A slow sampling scope would miss the spike for sure. A transient protection diode may be faster than the internal circuit?

SymTech Laboratories wrote:No one is sure that the flyback is what's damaging these VND5N07's anyway.

Launching a theory here: Although the VND is internally ESD protected there are limits to how much they can take. Is there some common point in time and/or space all the failed components share? Could they have been mishandled somewhere before installation?

subwoofer wrote:Launching a theory here: Although the VND is internally ESD protected there are limits to how much they can take. Is there some common point in time and/or space all the failed components share? Could they have been mishandled somewhere before installation?

That's certainly a possibility, but we've seen failed VND5N07's on more than just the MS3X. As far as we know, they were all driving high impedance fuel injectors when they failed.

Keithg wrote:I found mine when I probed the gate. The fet driver, IXDN404, was not able to drive the gate any more. My schematic has a pull down resistor on the gate and a resistor inline to the gate from the fet driver. When it failed, I was in 3rd gear going about 15 mph. I pressed on the accelerator and then it was running on 3 cylinders. So, the PW was low then went high when they failed. Mine failed and never came back. These are Bosch -431 HiZ injectors.

Peter Florance wrote:Can you measure the gate to source resistance of the dead device?

We have a dead VND5N07 here, and the gate-to-source resistance is ~49k ohms, just like the working ones; so the status feedback circuit isn't enabled if that's what you're asking. It's interesting to note that the MOSFET's body diode seems to shorted: the Rd-to-s and Rs-to-d are both in the 200k ohm range. Perhaps there's a negative voltage at the drain causing the diode to conduct (e.g. flyback recovery overshoot).